1. Field of the Invention
This invention relates to the preparation of racemic 1-phenylethylamine. More particularly, this invention is directed to the economic use of an optically active antipode of phenylethylamine and to a simple process for converting the same to the racemate whereby through resolution thereof the other optically active form can be readily prepared. This invention is particularly directed to the racemization of phenylethylamine by contacting an optically active form thereof with sodium amide or sodium hydride.
2. Discussion of the Prior Art
The antipodes of 1-phenylethylamine are being used increasingly as resolving bases for racemization since phenylethylamine is synthesizable and can readily be resolved into its antipodes. In contrast to naturally occurring alkaloids the optically active forms of 1-phenylethylamine can be provided in any desired amount for technical purposes.
In the racemization of synthetically prepared resolving bases, two antipodes are necessarily produced in equal amounts. However, it is largely only one resolving base that is needed for the racemization so that the other antipode is not needed at all. It is therefore necessary to re-racemize the antipodes of 1-phenylethylamine which are usually the necessary result of its production and to recycle them to the resolving process.
The racemization of optically active isomers is generally known. It is especially easy to racemize compounds which are capable of transformation to the enol form. Such substances have an activated hydrogen atom adjacent to the carbonyl group. By heating the substance or by warming it in the presence of an alkali or an acid such optically active compound can easily be transformed to the racemate.
Optically active forms of 1-phenylethylamine, however, do not have the structural characteristics of those optically active isomers which can readily be racemized, i.e., they do not have an activated hydrogen atom adjacent to a carbonyl group. In view of this chemical feature thereof the racemization of an optically active form of 1-phenylethylamine is particularly difficult. It has no electronegative substituents such as an aldehyde, ketone, acid or ester group, for example, which might give mobility to the hydrogen atom adjacent a carbonyl group.
In U.S. Pat. No. 3,168,566 there is described a process for the racemization of optically active antipodes which are difficult to transform into the racemate. There is particularly described therein a process for racemization of .alpha.-phenylamine and .alpha.-naphthylamine. In the process described the optically active amines are racemized by heating them, in substance, under an inert gas or in a diluent in the presence of an alkali metal. To achieve as complete a racemization as possible, the substance is heated up to about 200.degree. C. The best results are achieved with the addition of metallic sodium. Other additives are mentioned in the patent, notably aluminum isopropylate and sodium ethylate. These have proven to have little or no activity.
The process described in U.S. Pat. No. 3,168,566 suffers from some important disadvantages. Even on a laboratory scale a shift to larger units of volume than those specified in the patent results in a longer detention of the metallic sodium in the reaction mixture, i.e., heating time is increased, reaction time is increased and cooling time is increased. The result of this longer detention of metallic sodium in the reaction mixture is a decomposition of the amine due to the aggressiveness of the alkali metal. Consequently, great losses of yield result as shown in the comparative example below.
It has, therefore, become desirable to provide a process for the racemization of optically active forms of 1-phenylethylamine by a process which can be carried out on a large commerical scale. It has become particularly desirable to provide such a process which does not require the use of an alkali metal which always presents difficulty in handling. It has become desirable to provide a simple process which utilizes readily available common chemicals and which provides high yields of the desired racemate.